vmstate: Add support for alias ID

Some legacy users (mostly PC devices) of vmstate_register manage
instance IDs on their own, and that unfortunately in a way that is
incompatible with automatically generated ones. This so far prevents
switching those users to vmstates that are registered by qdev.

To establish a migration path, this patch introduces the concept of
alias IDs. They can be passed to an extended vmstate registration
service, and qdev provides a set service to be used during device init.
find_se will consider the alias in addition to the default ID. We can
then start generating the default ID automatically and writing it on
vmsave, thus converting that format without breaking support for upward
migration.

The user is required specify the highest vmstate version for which the
alias is required. Once this version falls behind the minimum required
for a specific vmstate, an assertion triggers to motivate cleaning up
the obsolete alias.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/hw/qdev.h b/hw/qdev.h
index d8fbc73..a44060e 100644
--- a/hw/qdev.h
+++ b/hw/qdev.h
@@ -44,6 +44,8 @@ struct DeviceState {
     QLIST_HEAD(, BusState) child_bus;
     int num_child_bus;
     QLIST_ENTRY(DeviceState) sibling;
+    int instance_id_alias;
+    int alias_required_for_version;
 };
 
 typedef void (*bus_dev_printfn)(Monitor *mon, DeviceState *dev, int indent);
@@ -112,6 +114,8 @@ int qdev_device_help(QemuOpts *opts);
 DeviceState *qdev_device_add(QemuOpts *opts);
 int qdev_init(DeviceState *dev) QEMU_WARN_UNUSED_RESULT;
 void qdev_init_nofail(DeviceState *dev);
+void qdev_set_legacy_instance_id(DeviceState *dev, int alias_id,
+                                 int required_for_version);
 int qdev_unplug(DeviceState *dev);
 void qdev_free(DeviceState *dev);
 int qdev_simple_unplug_cb(DeviceState *dev);